This invention relates generally to semiconductor device fabrication, and more particularly to the use of photoresist reflow in conjunction with assist features and/or off-axis illumination (OAI) for such fabrication.
Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC""s with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC""s have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. The resulting semiconductor features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. These distortions include line-width variations dependent on pattern density, which affect a device""s speed of operation, and line-end shortening, which can break connections to contacts. To avoid these and other optical proximity effects, the semiconductor industry has attempted to compensate for them in the photomasks themselves, as well as by using other approaches.
This compensation in the masks themselves is generally referred to as optical proximity correction (OPC). The goal of OPC is to produce smaller features in an IC using a given equipment set by enhancing the printability of a wafer pattern. OPC applies systematic changes to mask geometries to compensate for the nonlinear distortions caused by optical diffraction and resist process effects. A mask incorporating OPC is thus a system that negates undesirable distortion effects during pattern transfer. OPC works by making small changes to the IC layout that anticipate the distortions. OPC offers basic corrections and a useful amount of device yield improvement, and enables significant savings by extending the lifetime of existing lithography equipment. Distortions that can be corrected by OPC include line-end shortening, corner rounding, and isolated-dense proximity effect.
Isolated-dense proximity effect, or bias, in particular refers to the degree to which the mean of measured dense features differs from the mean of like-sized measured isolated features. Isolated-dense bias is especially important in the context of critical dimensions (CD""s), which are the geometries and spacings used to monitor the pattern size and ensure that it is within the customer""s specification. CD bias, therefore, refers to when the designed and actual values do not match. Ideally, bias approaches zero, but in actuality can measurably affect the resulting semiconductor device""s performance and operation. Isolated features, such as lines and contacts, can also negatively affect depth of focus (DOF), such that they cannot be focused as well with the lithography equipment as can dense features.
Contacts are two-dimensional features that are typically, but not necessarily, substantially square semiconductor features. They generally allow external electrical connectivity to semiconductor devices. Whereas OPC can improve resolution and depth of focus (DOF) for dense arrays and groupings of contacts, it is not as successful for random, isolated, and semi-dense contacts, which are generally referred to herein as non-dense contacts. Random contacts are those that appear randomly isolated within a semiconductor design. Isolated contacts can more generally appear either randomly or on an orderly or regular basis within a design. Semi-dense contacts are those that appear with a periodicity less than a given threshold.
OPC can be used to correct the isolated-dense proximity effect and the isolated-feature DOF reduction by adding scattering bars (SB""s) and anti-scattering bars (ASB""s) near the edges of opaque and clear features, respectively, on a photomask. SB""s are sub-resolution opaque-like features, whereas ASB""s are sub-resolution clear-like features. SB""s and ASB""s are specific examples of assist features, which are features added to the mask that are not specifically part of the intended semiconductor design, but which assist the proper imprinting of the design on the photoresist. Both SB""s and ASB""s serve to alter the images of isolated and semi-isolated lines to match those of densely nested lines, and improve DOF so that isolated lines can be focused as well as dense lines can with the lithography equipment. For example, FIG. 1A shows a set of SB""s 100, whereas FIG. 1B shows the placement of such sets of SB""s 100 near an isolated line 102, in contradistinction to the dense sets of lines 104 and 106.
Another issue that impacts the quality of lithography is focus variation, which is nearly ubiquitous in IC manufacturing because of the combined effects of many problems, such as wafer non-flatness, auto-focus errors, leveling errors, lens heating, and so on. A useful lithographic process should be able to print acceptable patterns in the presence of focus variation. Similarly, a useful lithographic process should be able to print acceptable patterns in the presence of variation in the exposure dose, or energy, of the light source being used. To account for these simultaneous variations of exposure dose and /focus, it is useful to map out the process window, such as an exposure-defocus (ED) window, within which acceptable lithographic quality occurs. The process window for a given feature, with or without OPC to compensate for distortions, shows the ranges of exposure dose and DOF that permit acceptable lithographic quality.
For example, FIG. 2 shows a graph 200 of a typical ED process window for a given semiconductor pattern feature. The y-axis 202 indicates exposure dose of the light source being used, whereas the x-axis 204 indicates DOF. The line 206 maps exposure dose versus DOF at one end of the tolerance range for the CD of the pattern feature, whereas the line 208 maps exposure dose versus DOF at the other end of the tolerance range for the CD of the feature. The area 210 enclosed by the lines 206 and 208 is the ED process window for the pattern feature, indicating the ranges of both DOF and exposure dose that permit acceptable lithographic quality of the feature. Any DOF-exposure dose pair that maps within the area 210 permits acceptable lithographic quality of the pattern feature. As indicated by the area 210, the process window is typically indicated as a rectangle, but this is not always the case, nor is it necessary.
Unfortunately, the process window typically varies by pattern feature. For example, the shape of the ED window for dense patterns, such as dense groupings of lines and contacts, is different than that for isolated patterns, such as isolated single lines and contacts. This is usually true even if the patterns have been modified by OPC to compensate for distortions. Individually optimizing the CD""s of a wafer""s features via OPC thus does not result in a maximized common process window over all the features. For various patterns, each having a different pitchxe2x80x94which is generally defined as the periodicity of a common feature within the pattern, such as a line or a contactxe2x80x94this means that applying OPC to each pattern to achieve identical pattern CD""s results in unequal process windows. The unequal process windows cannot be matched to one another to create a maximized common window.
At best, a non-maximized common process window still provides leeway as to varying exposure dose and DOF for obtaining an acceptable lithographic image. At worst, and perhaps more often, the non-maximized common window is more constraining, such that only within a narrow range of exposure dose and DOF will an acceptable lithographic image result. Fabricating semiconductor devices under such constraints is more difficult, and can result in significant wafer scrap and/or reduced fabrication yield and performance. Since most sub-wavelength lithography involves patterns having varying pitch, this can be a serious issue to modern semiconductor device fabrication.
Besides OPC, another approach that can be used to improve patterning is off-axis illumination (OAI). OAI is the shifting of the direction of the exposure beam during lithography from perpendicular, which interrupts the interference pattern that causes standing waves in the underlying photoresist being patterned. OAI particularly has the ability to significantly improve both the resolution and DOF for a given optical lithographic technology. For dense features, especially those having line-to-space duty ratios on the order of 1:1 to 1:3, such improvements are straightforward. Performance improvements are realized when illumination is obliquely incident on a mask at an angle so that the zeroth and first diffraction orders are distributed on alternative sides of the optical axis.
Examples of OAI are shown in FIGS. 3A and 3B. In FIG. 3A, the original center of illumination 302 has an illumination mask 304 positioned thereover. When an off-axis light source is instead used for illumination through the mask 304, first diffraction orders 306a and 306b result. The OAI of FIG. 3A is referred to as conventional OAI because the illumination mask 304 has a standard disc shape. In FIG. 3B, the original center 302 has an illumination mask 310 positioned thereover, resulting in diffraction orders 312a and 312b. The OAI of FIG. 3B is referred to as annular OAI because the illumination mask 310 has a ring shape. Other types of OAI include dipole, quadrupole, and quasar, which vary from one another and from conventional and annular OAI based on the illumination mask shapes used in such types of OAI.
Unfortunately, OAI does not measurably improve the process window, resolution, or DOF for isolated features, and specifically for isolated features that are contacts. This is because discrete diffraction orders do not exist for isolated features; rather, a continuous diffraction pattern is produced. The frequency separation of the diffraction orders is not great enough to improve performance of photolithography for isolated features by using OAI. Therefore, there is a need for improving the process window for non-dense contacts, so that the process window is comparable to that for dense contacts. Such process window improvement should also improve the resolution of these types of contacts, as well as the DOF for them. For these and other reasons, there is a need for the present invention.
The invention relates to photoresist reflow for an enhanced process window for non-dense contacts. A corrective bias is determined for application to each of a number of contacts at different pitches, to achieve a substantially identical critical dimension for each contact. The corrective bias is determined based on a first and a second critical dimension for each contact, where the first critical dimension is before photoresist reflow, and potentially inclusive of optical proximity effects, and the second critical dimension is after photoresist reflow. A photomask is then constructed for a semiconductor design that incorporates the corrective bias that has been determined for the contacts of the design. Lithographical processing of the semiconductor design on a semiconductor wafer using thus photomask, and subsequent photoresist reflow, thus achieves a substantially identical critical dimension for each of the contacts of the semiconductor design.
The invention provides for advantages not found within the prior art. Semiconductor devices fabricated using the invention have features with substantially the same critical dimensions, regardless of whether the features have different pitches, and thus regardless of whether the features are dense or non-dense. In other words, an overlapping process window is achieved for dense and non-dense features, such as contacts. This can enhance the resolution and the depth of focus (DOF) of the features. Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.